The present article is not only interesting because it presents and discusses the complex embryology of the atlas and axis vertebra as well as the possible origin of the os odontoideum. It also makes clear how dangerous the presence of such an os odontoideum is. The os odontoideum always goes along with a more or less pronounced atlanto-axial instability and represents basically a life-threatening condition. Such a malformation is often detected accidentally after any trauma leading to an X-ray of the neck. Unlike in the case presented here, the patients often are asymptomatic of the os odontoideum itself and therefore not very motivated to undergo spine surgery. The problem of the spine surgeon therefore is to persuade the patient and its family that this condition is really dangerous as long as not stabilized surgically. Any accident may be the cause of an acute tetraplegia or even death. Furthermore, in elderly patients with os odontoideum having survived without major event, a cervical myelopathy due to chronic compression trauma of the spinal cord is a frequent finding. Therefore, a posterior fusion C1/C2 should always be recommended as a prophylactic procedure, especially in young and active individuals, in whom the risk of an accident is certainly greater than in elderly patients.